1. Field of the Invention
The present invention relates to an air conditioner equipped with a scroll type compressor and a brushless DC electromotor serving to drive said compressor.
2. Description of the Prior Art
The typical conventional air conditioner for control of the room temperature has usually employed the compressor of reciprocating type or rolling piston type as the compressor for coolant used in the refrigerating cycles and the induction motor as the electromotor to drive this compressor.
However, the compressor of reciprocating type or rolling piston type exhibits a large compressive torque variation with respect to the turning angle, as indicated by characteristic curves a, b in FIG. 11 of the accompanying drawing, so such compressor has had to be provided with a large inertia mass in order to smooth a driving torque. Note that the characteristic curve a corresponds to the reciprocating 2 cylinder type while the characteristic curve b corresponds to the rolling piston type. The compressors of both types have inevitably been subject to a significant efficiency reduction during a high velocity operation, since their constructions have necessarily resulted in relatively high sliding velocities of the respective sealing parts. In addition, a starting torque of the induction motor has usually been limited to a relatively low level and, to overcome this problem, there has been proposed the countermeasure such that, during a restarting of the compressor, the latter is forcibly braked for several minutes until a desired balance is established between the high pressure side and the low pressure side or a pressure balancing is achieved by use of a bypass valve. However, the forcible braking has disadvantageously deteriorated the controllability and use of the bypass valve has unacceptably increased the energy loss.
Thus, the enclosed motor compressor employing the scroll type compressor has recently been used in practice. This scroll type compressor comprises a plurality of compression chambers provided between a pair of scrolls adapted to be moved eccentrically with respect to each other and, as indicated by a characteristic curve c in FIG. 11, exhibits a compressive torque variation relative to the turning angle, which is substantially smaller than those in the compressors of other types, enabling a restarting of the compressor to be achieved by a relatively small motor torque. In this compressor, moreover, the compression cycles continuously occur and, therefore, a differential pressure between each pair of adjacent compression chambers is correspondingly reduced. This feature is advantageous in that a possible gas leakage during each compression cycle may be reduced and the compression efficiency may be improved by 10% or higher in comparison to the compressors of other types. Furthermore, the respective slidably movable parts locally perform only their orbital movement and the sliding velocities of the sealing parts are correspondingly reduced, making high velocity rotation and, therefore, miniaturization of the compressor possible.
This particular scroll type compressor of prior art is advantageous also in that compression of coolant can be achieved without provision of a non-return valve on the inlet side or the outlet side for coolant, because a plurality of independent compression chambers are defined between said inlet side and said outlet side for coolant and thereby a quantity of coolant is prevented from flowing backward. Nevertheless, after the compressor has ceased to be driven, a quantity of coolant flows reversely from the compression chambers of the outlet side towards the compression chambers on the inlet side under a high coolant pressure on the outlet side, causing the movable scroll member to be reversely rotated. As a result, the direction in which the stationary scroll member and the movable scroll member should be otherwise loaded varies and the locations at which these scroll members should wear are displaced from each other. This event may often provide undesirable effects, for example, produce noisy sound and deteriorate the durability.
In such case, it might be conceivable to lack the rotor of the induction motor directly coupled to the movable scroll member in an electrical manner and thereby to block the movable scroll member against its reverse rotation. However, the principle of driving peculiar to the induction motor in general makes it extremely difficult to lock the rotor of the induction motor in the electrical manner.
It is for this reason that the scroll type compressor of prior art has been provided on the inlet or outlet side (often on the outlet side) for coolant with a non-return valve to prevent the movable scroll member from being reversely rotated when the compressor is deenergized.
In spite of such countermeasure, the maximum in- and output efficiency of the electromotor is in order of 70%, since the conventional air conditioner comprises a combination of the scroll type compressor and the induction motor. With a consequence, the output torque of the electromotor is lower than that obtained from the electromotor utilizing a permanent magnet. To assure an adequate starting torque, it will be inconveniently necessary to employ a large-sized electromotor. As an additional inconvenience, the previously mentioned manner, i.e., forcible lock of the induction motor must be still relied to obtain a desired pressure balance for the restarting at the cost of the controllability. Furthermore, even if the scroll type compressor adapted to be rotatable at a high velocity is employed to decrease a discharge per each rotation and thereby to reduce both the volume and the weight of the compressor section, 80 to 90% of both the volume and the weight has usually been occupied by the electromotor section in the enclosed motor compressor and accordingly miniaturization of the compressor section by adoption of the scroll type compressor has not necessarily brought as immediate result on the compressor as a whole. Also with respect to the efficiency( it has been impossible to make the best of a high compression efficiency proper to the scroll type compressor and the motor efficiency has governed an efficiency of the entire cooling system.
Additionally, the scroll type compressor has required to be provided either on the inlet side or on the outlet side for coolant with a non-return valve, as has previously been mentioned, and such requirement has necessarily led to a problem that a coolant flow resistance is increased by the presence of said non-return valve and a volume efficiency is thereby decreased during a high velocity driving of the compressor, precluding a desired improvement of the compressor performance.
In view of these problems which have been encountered by the conventional air conditioner, the present invention has been made to solve them efficiently.